A contrast of Pb(II), Cd(II), and Cu(II) toxicities to Aspergillus niger through biochemical, morphological, and genetic investigations

The toxicity of metals to microorganisms is highly correlated with the type of metal used. However, the differences in the resistance mechanisms of filamentous fungi to multiple metals remain unclear. In this study, we investigated the responses of Aspergillus niger to three toxic metals, i.e., Pb2+, Cd2+, and Cu2+. Fungal growth and metabolism indices showed that A. niger had a higher tolerance to Pb2+ (>1000 mg L-1) than to Cu2+ (300 mg L-1) and Cd2+ (50 mg L-1). An appropriate Pb2+ concentration (<500 mg L-1) stimulated fungal growth and metabolic activity, whereas Cd2+ and Cu2+ stress showed continuously negative influences on fungal physiological parameters, such as biomass and secretion of oxalic acid. A. niger responded to Pb stress by constructing a new border layer around its cell wall. This pathway was also confirmed using RNA-seq analysis, i.e., the gene encoding cell wall α-1,3-glucan synthase was upregulated. This upregulation subsequently promoted the production of polysaccharides, which are the main components that support fungal cell walls. In contrast, the expression of genes encoding both AAA family ATPase and efflux pump antibiotic resistance proteins for Cd2+ and Cu2+ was significantly downregulated. Therefore, these findings elucidated the relatively complete fungal responses to different metal stresses.